In recent years, power generation efficiency has been improved with respect to energy harvesting devices such as solar batteries, thermoelectric generators, or vibration-powered generators, which generate electric power from ambient energy. MCUs (micro-control units) operable with minute electric power have also been developed. An MCU and an energy harvester may be combined to implement an embedded device that is cable of operating without receiving power from an external source. One of the applications of such embedded devices may be a wireless sensor node. The realization of a wireless sensor node capable of operating without receiving electric power allows a wireless sensor node to be installed in an unsafe high place, a place where toxic gas is present, a place where temperature and humidity are unbearable for men, etc., without the need for manual replacement of a battery, and is thus extremely useful.
Despite the improvement of power generation efficiency, the power consumption of an MCU used in an embedded device needs to be reduced as much as possible in order to allow the embedded device to operate with the limited power derived by an energy harvester. In a wireless sensor node, the length of an idle state that is a waiting state preceding the start of a next operation is extremely long, compared with the length of an active state during which a sensing operation, a wireless communication, and so on are performed. In the case of such an embedded device, reducing power consumption in idle states, i.e., in sleep modes, is an important factor to be considered in order to reduce the average of total power consumption. Reducing the MCU's power consumption in a sleep mode involves setting appropriate signal input and output modes to its I/O ports so as to reduce electric currents flowing between these I/O ports and another IC.
Integrated circuits available today such as MCUs designed to have low power consumption include those which allow the signal input and output modes of its I/O ports to be freely set according to software-based control or the like. Each of the I/O ports of such an integrated circuit is provided with a plurality of signal input and output modes, one of which is selected and set according to control by software or the like operating in the integrated circuit. The signal input and output mode set to an I/O port during the active mode according to control by software or the like stays unchanged even after a transition is made from the active mode to the sleep mode. On the other hand, a normal integrated circuit that is not specifically designed as the low power integrated circuit as described above has an I/O port that is automatically set to a high-impedance state in the sleep mode.
Connecting the normal integrated circuit and the low power integrated circuit through their I/O ports with each other results in the flow of needless steady current unless the I/O ports are set to proper signal input and output modes by taking into account various combinations among the active mode and the sleep mode.
[Patent Document 1] Japanese Laid-open Patent Publication No. S63-36350
[Patent Document 2] Japanese Laid-open Patent Publication No. H1-251148